1. Technical Field
The present invention relates to technical fields of surface emission device, optical element and liquid crystal display device. For more detail, the present invention relates to technical fields of suppressing non-uniformity in luminance by allowing split images of light sources to overlap between the light sources.
2. Background Art
Liquid crystal display devices provided with backlight (surface emission device) have conventionally been used as display devices for word processors, laptop personal computers and so forth. As the surface emission device for this type of liquid crystal display devices, an edge-light-type backlight, having linear light sources just like fluorescent lamps disposed laterally on a transparent plate (light guide plate), in response to demands for weight reduction and thinning, has been in the main stream.
The edge-light-type backlight has, however, often resulted in insufficient luminance with recent expansion in size of the liquid crystal display device represented by those used for television sets, so that a direct-type backlight, having linear light sources arranged straightly under the liquid crystal display panel has more widely been adopted.
FIG. 32 is a perspective view showing a schematic configuration of a conventional direct-type backlight unit 1. The backlight unit 1 has light sources (linear light sources) 2, 2, . . . such as fluorescent lamps, a reflective plate 3, and a diffuser plate 4.
As the light sources (linear light sources) 2, 2, . . . , cold cathode fluorescent lamps (CCFL) or the like are used, which are formed into columns extended in a predetermined direction.
The reflective plate 3 is disposed so as to make use, in a recycled manner, of light reflected on the diffuser plate 4, etc., or light emitted from the light sources 2, 2, . . . , but not reached the diffuser plate 4.
The diffuser plate 4 is an optical element of at least 1 mm thick or more improved in diffusing and scattering performances, by virtue of having a transparent base and a resin component different from the transparent base in the refractive index randomly contained therein, and is used as an optical element for suppressing variation in front luminance distribution.
In the backlight unit 1, the reflective plate 3 and the diffuser plate 4 are disposed respectively on both sides of the light sources 2, 2, . . . .
In thus-configured backlight unit 1, light emitted from the light sources 2, 2, . . . is extracted from the diffuser plate 4, wherein luminance of illumination flux of the backlight unit 1 may be high straightly above the light sources 2, 2, . . . and may be low between the light sources 2, 2, . . . as shown in FIG. 33, when the distance between the light sources 2, 2, . . . and the diffuser plate 4 becomes small, or the distance between the individual light sources 2, 2, . . . becomes large, and this may degrade uniformity in the front luminance distribution and may cause variation in luminance.
In order to suppress such variation in luminance, as shown in FIG. 34, there has been known a technique of disposing an optical sheet (optical element) 5 such as prism sheet or lenticular lens sheet between the light sources 2, 2, . . . and the diffuser plate 4, or disposing an optical sheet (optical element) 5 such as prism sheet or lenticular lens sheet in place of the diffuser plate 4 (see Japanese Patent Application Publication (KOKAI) Nos. H5-333333, H6-250178, H10-283818, and 2004-6256). FIG. 34 shows an exemplary case where the optical element (prism sheet) 5 is disposed in place of the diffuser plate 4 shown in FIG. 33.
The optical element (prism sheet) 5 has, on the front surface or the back surface thereof, a plurality of linear projections (prisms) consecutively provided at regular pitches, typically having a triangle profile, and is an optical element generally adopted as a sheet for improving luminance. These linear projections function as a luminance distribution generating layer 5a which suppresses variation in luminance in the direction of optical axes of light emitted from the light sources 2, 2, . . . .
The optical element 5 is disposed so that the direction of ridge of the linear projections which function as a luminance distribution generating layer 5a agrees with the longitudinal direction of the light sources 2, 2, . . . . By using the optical element 5, as shown in FIG. 34, extracted illumination flux is split into a plurality of fluxes to give split images 2A, 2A, . . . of each light source, and thereby the variation in the front luminance distribution may be suppressed. FIG. 34 shows an exemplary case where the number of split images 2A, 2A, . . . of each light source was doubled from the number of the light sources 2, 2, . . . by the optical element 5.
The above-described conventional surface emission device 1 has, however, been suffering from a problem in that a large non-uniformity in luminance is likely to occur, when the distance between the light sources 2, 2, . . . and the optical element 5 varies. Variation in the distance may be ascribable to accuracy in processing or assembling of the individual components, or to deformation of the optical element due to environmental changes such as changes in temperature.
For example, as shown in FIG. 35, in a surface emission device designed to ensure uniform front luminance distribution with respect to each of the split images 2A, 2A, . . . of the light sources 2, 2, . . . , when the distance between the centers of the light sources 2, 2, . . . and the optical element 5 is given as H, a change in the designed distance H of the optical element 5 to as much as ΔH may be highly causative of non-uniformity in luminance as shown in FIG. 36.
The surface emission device 1 is designed so as avoid overlapping of one split image 2A of the light source 2 with the adjacent split image 2A of the light source 2, so far as the designed distance H is maintained, so that such non-uniformity in luminance may occur as a result of a sharp change in the front luminance distribution when the distance H varies. More specifically, a change in the distance H to as much as ΔH may cause overlapping of the split images 2A, 2A, . . . of the individual light sources 2, 2, . . . , and may raise a sharp change in the front luminance distribution, making non-uniformity in luminance more likely to occur.
The distance between the light sources 2, 2, . . . and the optical element 5 is therefore designed so that a uniform front luminance distribution, as shown in FIG. 35, may be obtainable, but only with a small degree of freedom in the design.
On the other hand, with recent trends in expansion in size of the liquid crystal display devices, also the surface emission devices (backlight unit) have been expanded in size. As a consequence, also the optical elements such as prism sheet, lenticular sheet and so forth are to be expanded in size, for the purpose of making the front luminance distribution uniform.